The present invention generally relates to a process for preparing esters of S-alkylthiopropionic acids and to the esters thus produced. More particularly, the invention relates to esters of 3-alkylthiopropionic acids with pentaerythritol or the like which are purified with a solvent blend and which are esters having an extremely high level of tetraester structure.
Alkyl esters derived from alkylthioalkanoic acids and the like are, in general, known to be useful as stabilizers of organic materials such as polymer resins and the like which are otherwise subject to thermal and oxidative deterioration during processing, extrusion or molding, as well as during use. Esters having this general utility have in the past been prepared by various procedures. Dexter et al U.S. Pat. No. 3,758,549, for example, basically teaches transesterification procedures for the preparation of these types of products. By such procedures, it is often difficult to obtain a product that has a tetraester content at or above 90% by weight, particularly when the transesterification is carried out on an industrial scale.
Stabilizers for enhancing the resistance of polyolefins to deterioration can also be prepared by reacting an alpha-olefin with a multi-functional ester of a mercaptocarboxylic acid. Stabilizers of this type and the process for their preparation are disclosed in Kauder et al U.S. Pat. No. 4,080,364. Experience with this type of addition reaction indicates the product thus formed has a tetraester content which typically does not meet or exceed 90% by weight.
Nakahara et al U.S. Pat. No. 4,349,468 teaches the preparation of a pentaerythritol tetrakis (3-laurylthiopropionate) stabilizer for polyolefins which is produced by a process including heating an alpha-olefin such as 1-dodecene with a beta-mercaptopropionic acid or ester in the presence of an azonitrile or peroxide catalyst, followed by esterifying the resultant alkylthiopropionic acid with pentaerythritol. The resulting product is typically inferior in that the alpha-olefin reaction produces an unwanted isomer byproduct that, if not removed in a separate purification step, lowers the quality of the pentaerythritol ester.
Alkylthiopropionic acids prepared by approaches such as these may be subjected to solvent refining according to the present invention. However, they do not typically directly produce, without special purification, an alkylthiopropionic acid which exhibits a high purity characteristic which will, when reacted with pentaerythritol or the like, form an ester product which has an extremely high tetraester content and a minimum of other components such as the triester. At times, a substantially high excess of the acid reagent is required, which is costly and inefficienct and can require removal of excess acid from the reaction product and purification at an intermediate stage.
It has been determined that a 3-alkylthiopropionic acid having about 4 to about 20 carbon atoms in the alkyl group can be prepared by directly reacting an alkyl mercaptan having between about 4 and about 20 carbon atoms with an alkali metal acrylate, the reaction being carried out in the presence of a strong base catalyst, typically within an aqueous alkaline solution having a pH of at least about 11. Thereafter, the reaction solution is acidified to a pH which is at about 4 or below, and the 3-alkylthiopropionic acid is then recovered from the water-insoluble phase of the acidified reaction solution. A tetraester product is then formed by esterification of the 3-alkylthiopropionic acid with pentaerythritol or the like.
By the present invention, a tetraester prepared in this manner or in any other suitable manner is subjected to a purification procedure that has been found to be especially effective and efficient in removing unreacted acid and tris ester components from the tetraester product. The tetraester product is solvent refined with a blend of at least two organic solvents. Certain solvent blends are especially well suited for particular ester products.
It is accordingly a general object of the present invention to provide an improved process for preparing an alkylthiopropionic acid ester.
Another object of this invention is to provide an improved 3-alkylthiopropionic acid ester of particularly high purity.
Another object of the present invention is to provide an improved process for the work-up purification of a tetraester formed from a 3-alkylthiopropionic acid and pentaerythritol.
These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description.